Atoms and Molecules

The Particles of Matter and John Dalton

The idea that the world around us is made up of large numbers of identical
very small particles called molecules, and that the myriads of different
kinds of molecules are simply differently arranged groups of atoms, is
only a little over 150 years old. Yet it is in terms of this picture that
any educated person views the world, and it is this picture that lies at
the heart of chemistry.

The properties of both compounds and elements, especially the relative
masses of the different elements which made up a compound, were studied
extensively in the eighteenth century. Correspondence between chemists,
formalized by publication of experimental results in the journals of scientific
societies, led to the compilation of tables of composition and properties
of pure compounds. It remained for the Quaker schoolmaster John Dalton
to place these painstakingly collected measurements and empirical laws
on the theoretical basis known as theatomic theory of matter.

The understanding of the nature of matter as molecules was first set
down by John Dalton (1766-1844). Dalton lived in Manchester, England, and
from 1793 earned his living as a private tutor there. In the course of
his studies and teaching, as well as in discussions with other members
of the Literary and Philosophical Society of Manchester, he developed the
ideas which led to his formulation of the atomic theory of matter in 1805.

Quantitative chemical measurements, especially those of mass, had given
rise to several empirical laws which preceded the theoretical basis given
to them by John Dalton. Dalton was familiar with these laws and also with
the laws which had been discovered to describe the behavior of gases as
we shall see in later sections. It was to explain these empirical laws
that he developed the atomic theory of matter.

Probably the best-known and most widely accepted of the emprical laws
known to Dalton was the law of conservation of (total) mass, which
had been used by A. L. Lavoisier in 1798.
It was probably first assumed, then verified by a few experiments, although
it can only be truly verified by very precise experiments using sealed
systems such as were carried out some fifty to one hundred years later.
It had generally been accepted by scientists, however, even prior to the
work of Lavoisier. The law of conservation of mass can be stated as follows:
The total mass of the reactants in any chemical reaction is exactly
equal to the total mass of the products.

The second empirical law known to Dalton was the law of constant
composition of compounds, most ably stated by J. L. Proust (1755 -
1826) in his publications in J. Physique (Madrid) in 1802 - 1808. This
law, clearly established by the long controversy between J. L. Proust and
C. Berthollet, was a statement of the observed fact that any pure substance
has a fixed composition in terms of the chemical elements.

For example, there are three compounds known which contain only iron
and oxygen. They have different chemical compositions and different physical
properties, but for each compound the properties, and the composition,
do not differ between samples obtained from Australia to Zanzibar or, indeed,
from the earth or the moon. In other words, the mass ratios or percentage
composition by mass of any compound is characteristic of that compound
and cannot be altered without changing the other properties which are also
characteristic of the compound. The law of constant composition of compounds
can be stated as follows: For every pure compound the mass ratios of
its constituent elements are constant.

The empirical law of multiple proportions was apparently developed
by Dalton himself around 1804. Again, it was probably assumed, then verified
by a few experiments; it became public in 1807. The law of multiple proportions
can be stated as follows: When any two elements are observed to form
more than one compound between them, the mass ratios in one compound will
be related to the mass ratios in the other in the proportions of small
whole numbers.

The determination of the empirical mass ratios in compounds should ideally
be done by decomposition to the elements (elemental analysis). However,
in the eighteenth and early nineteenth centuries it was often impossible
to effect the decomposition quantitatively, so many of the ratios were
determined by synthesis from pure elements. Prior to the development of
electrolysis, decomposition of water to hydrogen plus oxygen was not possible,
but preparation or synthesis of water from hydrogen and oxygen was possible.
When hydrogen was burned in excess oxygen water of constant composition
was always evolved. The water could be weighed directly by absorption,
the hydrogen used could be obtained by difference, and the oxygen used
was established assuming conservation of mass. It can then be calculated
that water is 11.19% hydrogen and 88.81% oxygen by mass. Dalton assumed
that hydrogen should be assigned an atomic mass of one because it was the
lightest element known. The mass of oxygen is then 88.81/11.19 = x/1, where
x is the atomic mass of oxygen, and the value of 7.937 obtained was rounded
to eight.

The decomposition of Fe2O3 was likewise impractical,
but the reaction (rusting or oxidation) of iron in the presence of excess
oxygen is simple and yields a compound of constant composition. The original
iron can be weighed, as can the resulting iron oxide. The oxygen can be
obtained by difference since conservation of mass is assumed. The compound
has the constant composition Fe 69.9%, O 30.1% by mass. The obviously different
compound FeO cannot be prepared by synthesis from the elements but must
be tackled by analysis: reduction with hydrogen, yielding water and iron
metal. From the mass of the original iron oxide, the mass of the water
produced, and the mass of the resulting iron, the composition by mass of
FeO is calculated to be 72.7% Fe and 27.3% oxygen. The atomic mass of iron
is then 72.7/27.3 = x/8, or 27.9 units.

The units of atomic mass are now called unified atomic mass units,
abbreviated u or amu. Biochemists often call them daltons in honor
of John Dalton.